The subject of this invention is a new process for detecting and analyzing the interaction of gamma and X-rays, particularly at high energies (in practice called hard X-rays) with an object or a patient (target under study further on).
It includes a detector which starts up this process, and a radiographic system, which reconstitutes in real time the detected image of an object or an X-rayed element, the incidental flux of which is made up of gamma photons or hard X-rays, thus activating the detector.
The generation of gamma rays and hard X-rays, typically with an energy of more than some 100 kiloelectronvolts (keV), is nowadays widely developed in order to study the internal structure of material objects or, in the sphere of radiotherapy, notably to treat cancers. However, the difficulty in obtaining images of quality that can be correctly interpreted is important, especially in comparison with traditional radiographical techniques using low energies.
These difficulties are not only inherent in the lower absorption coefficient of the gamma rays, they also come from the different phenomena of interaction that come into play. Indeed, when a beam of x- or gamma rays penetrates matter, there occurs a phenomenon of attenuation of energy of the incidental photons following the interaction between the photons and the matter they pass through (i.e., pass-through radiation).
This interaction can occur either with an electron of matter or with one of the nuclei of the atoms that make up this matter. One type of interaction that can be distinguished with electrons is the Compton effect, i.e. the ejection of the electron that was the object of the interaction, and the creation of a scattered photon; another is the photoelectric effect, i.e. the ejection of an orbital electron under the action of the energy transferred by the incidental photon. The photoelectric effect is dominant for energies of incidental photons of up to around 100 keV. Beyond this threshold, the photoelectronic effect diminishes in favor of the Compton effect.
The interaction with the nuclei leads to the creation of electron-positron pairs, this effect becoming dominating over the Compton effect beyond energies of the order of 5-10 MeV depending on the atomic number of the matter passed through.
The low absorption coefficient added to the fact that its value, specific for each atomic element, becomes very similar for these elements as soon as the energy of the incidental photons reaches about 1 MeV, leads to a significant reduction in contrast, limits the efficiency of the detectors of these rays integrated within the radiographic installations, and seriously limits the quality of the images that are captured.
Moreover, highly energetic electrons and high energy secondary photons interact with the experimental volume, making it necessary to use stronger barriers against these secondary effects, constituting as many sources of particles affecting the detected signal, notably its contrast and limiting the resolution in terms of position within the resulting image.
In industrial radiography, high energy X-rays are used to X-ray relatively dense objects, for example in order to carry out non-destructive testing of steel constructions, to perform tests on soldering, etc. X-ray imaging also provides information on the internal structure of the object.
This type of X-rays, situated in the same range of energy, are also used in radiotherapy for the treatment of malignant tumors. In this case, this radiation is used in order to modify the biological structure of living tissues and also to destroy them. The X-ray image of the irradiated patient helps the operator to improve the positioning of the patient and to better regulate the collimation diaphragms in order to achieve a better quality of treatment and especially to reduce the risks of lesions of healthy tissues by reducing as much as possible the dose delivered whilst checking the position.
Radiographic imaging using X-rays of such energies, i.e. higher than 500 keV, would appear to be difficult to perfect, taking into account the weak effective cross-section of the interaction of the photons with matter. In fact, images with weak contrast are generally obtained, taking into account the weak dependence of the attenuation coefficient on the photon with the atomic number, so that the regions constituted by different elements are distinguished with difficulty. In order to improve the quality of the images, it would therefore appear to be necessary to acquire a large sample of photons detected at the level of the detector. Thus, two possibilities present themselves. The first is to increase the dose of radiation administered to the object. The second is to optimize the efficiency of the detection of photons at the level of the detector.
If the first of these possibilities is entirely acceptable in the sphere of analysis of static objects, especially at the industrial level, this is certainly not the case in the context of radiotherapeutic treatment for which the dose administered is strictly controlled for obvious safety reasons.
Another important factor that reduces the quality of radiotherapeutic images when high energy photons are used is the large fraction of photons that come not from the principal source of photons, but which are secondary photons created by interactions such as the Compton effect or Bremsstrahlung, occurring as much in the matter surrounding the irradiated object, in the irradiated object itself, in the collimation elements, or even in the detector itself.
Finally, another factor contributing to the difficulty in obtaining images of good quality relates to the weak effective cross-section of the interaction of high energy photons, for which only a small fraction of the photons passing through the object to be analyzed contribute to the image.
One of the objectives of the invention is to optimize the efficiency of the detection of primary photons by the detector. To do this, it aims to selectively detect only the part of the spectrum of primary or direct photons by the use of a threshold, and thus to minimize the contribution of secondary or scattered photons to the signal.
A gamma ray or X-ray detector is placed behind the object or the patient to be treated. For industrial radiography, this detector should be able to provide an image of the internal structure of the irradiated part of the object with a maximum of precision and contrast. For radiotherapy, it must provide an image of sufficient quality with a minimum dose of radiation.
Among the best known detectors are those that use portable films made of an X-ray sensitive film sandwiched between two metallic plates. Despite a good efficiency at low energy and a good spatial resolution, this kind of system has a mediocre contrast.
Moreover, and above all, the use of these films makes it impossible to obtain images in real time or nearly real time, which is becoming more and more necessary in radiotherapy.
It therefore became necessary to develop an electronic imaging device for testing, of the type better known as xe2x80x9cElectronic Portal Imaging Devicesxe2x80x9d (EPID), especially on-line, able to deliver an image in real time at high contrast and using a weak dose administered to the patient.
Systems were proposed such as a video system with a mirror and phosphorescent screens. This system consists of a metallic plate coated with fluorescent phosphorus, the screen being visualized by a video camera using a mirror at an angle of 45 degrees. The interaction of the X-rays with the metal plate creates high energy electrons by photoelectric effects, the Compton effect and the creation of pairs, and induces fluorescence inside the screen.
Although this type of system develops a good spatial resolution, it has only a weak contrast and a high dose is often necessary to obtain a readable image. Moreover, these systems have a tendency to age relatively rapidly, are bulky and costly in production
Another device that was developed was an ionization liquid chamber. The ionization chamber consists of a matrix of wires composed of two parallel surfaces of wires, typically of 256 wires for each surface. The 256 electrodes of one orientation detect the signal, each of the electrodes being connected to a very sensitive current detector (picoamps). The other 256 electrodes work as high tension electrodes and are linked to a tension switch. Between the two surfaces is the ionisation liquid. The signal comes from ionization of the liquid in the form of electron-ion pairs and its amplitude is proportional to the energy deposited in the medium by the ionizing particles during the integration time of the read-out electronic circuit.
In fact, to obtain an image, the matrix is scanned line by line whilst switching the tension successively on each electrode.
Even though the resulting device is quite compact, and its sensitivity is relatively acceptable, there are difficulties in calibration of the matrix and the purity of the organic liquid is often difficult to obtain. Moreover and above all, the weak contrast does not make it possible to obtain images of very good quality.
Other systems have been developed using solid state technology, in particular silicon detectors. These detectors can be made of a linear array of 256 silicon diodes, placed next to a lead plate of about 1 millimeter thick, each silicon diode detecting the high energy electrons created by the X-rays that interact with the lead plate, and penetrating into the sensitive volume. The ionized charges in the depleted layer of silicon are amplified and digitalized by means of an analogue-digital converter.
Detectors of this type need in general high doses to provide contrasted images of sufficient quality and the total duration of verification of the fields is generally greater than for bi-dimensional imaging devices.
Finally, recent developments in the field of materials science and in the technology of hydrogenated amorphous silicon (a-Si:H) have made it possible to increase the pixel resolution of large photosensitive bi-dimensional detectors. An a-Si:H matrix is positioned immediately after a sandwich of a metallic plate and a phosphorescent screen, these two components fulfilling functions identical to those in systems with a fluoroscopic camera. In fact, the a-Si:H matrix serves as a substitute for the mirror, the camera with a lens, or even of a bundle of optical fibres, in classic systems. Compared with other optical systems, the advantage of using an array of photodiodes positioned near a phosphorescent screen is that a large fraction of the light emitted can be captured and converted into a signal. However, this type of system has the inconvenience of exposing the cards with very sensitive amplification electronics, which are directly integrated inside the array to intense fluxes of hard X-rays during the treatment sessions in radiotherapy, which are capable of inducing fluctuations in the signal and may cause radiation damage of the electronic channels.
The objective of the current invention is therefore to propose on the one hand a process and on the other a detector, and finally a system for radiographic imaging in real time which can discriminate low energy ionizing radiation, which consists at least in part of scattered radiation, and thus to eliminate their contribution to the detected image, so that the contrast is increased, and this can be done with relatively reduced doses of incidental radiation.
In the same way, the invention aims to improve the signal/noise ratio, and also the spatial resolution of the image thus acquired.
The process invented, and the functioning of the detector and of the system that activates it are based on the principle of the emission threshold for light of the Cherenkov type, which appears when the charged particles generated by the interaction between photons and matter pass through a specific dispersive medium.
This process to detect and analyse the interaction of the gamma and X-rays with an object consists in positioning next to this object subjected to the irradiation of incidental rays a first material capable of emitting, under the action of the radiation emerging from this object, high energy charged particles, then to interpose in the path of these particles a second material capable of emitting a Cherenkov radiation after interacting with the particles. The refraction index of the second material is selected in such a way that the Cherenkov emission could only occur for the specific energy threshold of these charged particles, i.e. the emerging rays, the Cherenkov emission being detected by means of a photon detector.
Thus, taking into account the fact that the Cherenkov emission occurs only for a specific energy threshold, it becomes possible to operate a selection by energy bands. In fact, one can eliminate the xe2x80x9cnoisexe2x80x9d that comes for example from low energy scattered gamma rays.
The incidental gamma and X-ray detector is constituted of:
a converter of the incidental rays into high energy electrons, created by phenomena of pair production and the Compton effect;
a photon emitter by the Cherenkov effect under the action of high energy electrons from the converter, this emitter being attached to the converter;
an element of detection sensitive to the photons thus created and capable of restoring spatially the emission density of the Cherenkov photons coming from the Cherenkov emitter.
It is better to interpose between the Cherenkov photon emitter and the detection element a layer of a material intended to shift the wave length of the Cherenkov photons emitted in the visible waveband.
For example, the converter is constituted of a material with a high atomic mass number, chosen from the group including tungsten, lead, copper, pure or in the form of an alloy.
The thickness of the converter is preferably in the range 0.1 to 20 millimetres in most cases.
The Cherenkov emitter has typically a refraction index of between 1 and 2. It is composed of an optically transparent material, in the form of a crystal, an amorphous solid or a liquid. It can be made, for example, of calcium fluoride, sodium fluoride, lithium fluoride, magnesium fluoride, or even of natural or synthetic silicon or of silicon aerogels. In the liquid state, it can be made of water, or even of freon.
The detection system is preferably composed of a camera with a charge coupled device (CCD) with or without an image intensifying device, or even with a cooling device for the CCD sensor. This camera is coupled optically by means of a mirror and a lens either directly to the Cherenkov emitter or to the wavelength shifter. This coupling can also be done by using a bundle of optical fibers.
The detection system can also be made of a matrix of photosensitive elements like bi-dimensional hydrogenated amorphous silicon ones.
The wave length shifter is preferably made of a layer of sodium salicylate of a thickness ranging between 10 nanometers and 500 micrometers covering the output surface of the Cherenkov emitter. It can also be selected from the group including sodium salicylate, p-terphenyl, diphenyloxazole (DPO), tetraphenylbutadiene (TPB), p-quaterphenyl (PQ), diphenylstilbene (DPS), trans-stilbene (TS), diphenylbutadine (DPB), phenylene phenyloxazole (POPOP), bis(2-methylstyryl)benzene (bis-MSB), benzimidazo-benziso-quinoline (BBQ).
A brief resume of the principle of Cherenkov emission is given hereunder:
A charged particle moving at a speed v=xcex2c (where xcex2 represents the number of units of the speed of light c in a vacuum), in an optical medium with a refraction index n, emits photons called Cherenkov photons when the speed v exceeds the speed of light in the medium, i.e. when v greater than c/n, or when xcex2 greater than 1/n.
The Cherenkov polar angle of emission in respect to the direction of the incidental particle is given by the equation : cos xcex8=1/Nxcex2.
The majority of photons is emitted in the wavelengths close to ultraviolet.
The way in which the invention can be applied and the advantages to be derived from it can be better shown in the following example together with the illustrative figures, given as an indication and not limitation.